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Archived Rebuttal
This is the archived Basic rebuttal to the climate myth "There's no empirical evidence". Click here to view the latest rebuttal.
What the science says...
|
There are multiple lines of direct observations that humans are causing global warming. |
The claim that there is no empirical evidence to support the role of humans in current climate change ignores the actual evidence that has been collected over the decades. Scientists have reached the conclusion that humans are most likely responsible for climate change by looking at all the pieces of the jigsaw and placing them in the context of our knowledge of physical processes. As a result, the evidence can be broken down in the following way.
Human activies are causing an increase in atmospheric CO2
Direct observations from around the world confirm that CO2 levels have been increasing since the 1950s, when instrumental measurements began. CO2 levels before this period can be determined by looking at air bubbles trapped in ice cores and have been found to be stable at about 275 to 285 parts per million (ppm) for the 10,000 years prior to the Industrial Revolution. Since then, levels have risen by over 100 ppm (~ 40%) and now stand at approximately 391 ppm.
But how do we know that human activities are responsible for this increase? First, it is possible to calculate the amount of CO2 released from international energy statistics. According to the US Energy Information Administration, CO2 emissions stand at 29.7 metric tons for 2007. The link between CO2 emissions from human activities and CO2 levels in the atmosphere is clearly visible in figure 1 below, which shows that atmospheric concentration started to rise after humans started to burn fossil fuels.
Another clue as to the origin of this additional CO2 comes from the analysis of different variants of carbon, known as carbon isotopes. Isotopes of a particular atom vary in the number of neutrons found in their nucleus. In the case of carbon, we are particularly interested in carbon-12 (C12) and carbon-13 (C13), which have 6 and 7 neutrons respectively. Both occur naturally in the atmosphere, though C12 is much more abundant. When photosynthesising, plants prefer to use C12. This means that the carbon sequestered in fossil fuels contains a lower ratio of C13 to C12 (C13/C12) compared to the atmospheric ratio. As a result, when we burn fossil fuels and release the carbon sequestered within them, we should see a corresponding change in the atmopheric C13/C12 ratio. This is exactly what is being observed: the atmospheric C13/C12 ratio is falling. This strongly suggests that the additional carbon is coming from the combustion of fossil fuels.
CO2 absorbs heat
CO2 is a greenhouse gas, which means that it absorbs part of the infrared radiation, i.e. heat, being emitted by Earth and in turn re-emits it in random directions. So instead of escaping into space, this heat is intercepted and retained within the system. Adding CO2 to the atmosphere should therefore lead to quantifiable changes, and these changes have been observed. First, scientists have used satellite data to look at changes in outgoing radiation between 1970 and 1996. As predicted, there was a drop in outgoing radiation at those wavelengths that CO2 and other greenhouse gases absorb energy.
Similarly, because part of the radiation re-emitted by the additional CO2 will travel downward, we should be able to detect an increase in the amount of infraradiation returning to the Earth's surface. Again, this has been observed in surface measurements from 1978 to 2008, with a more detailed analysis showing the contribution of each greenhouse gas to this increase in downward radiation.
Finally, in addition to this direct evidence for an impact of increasing CO2 levels, the human fingerprint on climate change is apparent in a number of observations, which further reinforces the case for a human role in climate change.
Updated on 2011-05-25 by Anne-Marie Blackburn.
THE ESCALATOR
(free to republish)